This invention is in the field of optics for implementing an optical depolarizer by means of a corner-cube.
Linearly polarized light launched into a fiber-optic invariably experiences birefringence as it propagates in the fiber. Birefringence effects transform linearly polarized light into elliptically polarized light that can compromise performance in signal processing, and in degrading performance in pumped optical amplifiers for long-haul communications systems. Depolarized light is not affected by birefringence, and thereby propagates without any polarization perturbations in the fiber. A depolarized light source is best described by the Stokes polarization parameters S1, S2, S3 having zero values.
Depolarized light sources are useful in minimizing adverse polarization problems of fading in signal processing. Also in fiber-optic communication systems depolarizers have become useful in pumped optical amplifiers. Most optical amplifiers are polarization dependent in gain which means, a polarized pump source enhances gain in signals having the same polarization state. Since the polarization state of the signal cannot be effectively controlled, non-uniform signal gain occurs due to the inherent time-varying changes in the polarization state of the beam in the fiber. For this reason a depolarized pump source is used for the amplifier because it provides uniform gain characteristics. Considerations have also been given in using a depolarized light source for the signal at the transmitter in communication systems thus eliminating signal polarization issues in the amplifier as well.
Many different types of depolarizers have been described in the literature with the aim in generating a sufficiently large number of polarization states in the output beam such that the sum of each Stokes polarization parameter cancels out to zero (except for S0=1). Depolarization can be performed by different methods as in 1) the electro-optic scrambler, 2) spatial averaging of a beam emerging from a retardation plate having phase gradient characteristics, 3) spatial averaging in quartz plate of the Lyot type, 4) re-circulation loop averaging by splitting and recombining light beams, and 6) spatial averaging of the retro-reflected beam from a high refractive index corner cube. Developments in various types of depolarizers will be reviewed and cited.
The polarization transformer as described in U.S. Pat. Nos. 4,966,431 and 5,212,743 (Heismann, dated Oct. 30, 1990 and May 18, 1993, respectively) can be used as a scrambler, also called a pseudo-depolarizer. The device consists of an electrically-controlled integrated-optic waveguide structure in a LiNbO3 substrate. The scrambler speed has to be fast enough such that the scrambled polarization states are blurred together into a time-averaged depolarization state without showing adverse polarization effects. Other types of depolarization systems have also been described. For example, depolarization can be achieved in a birefringent medium by re-circulating a split-off portion of the output light back into the input of the birefringent medium as described in U.S. Pat. No. 6,421,471 (Shen, Jul. 10, 2002). The output Stokes polarization parameters of the light beam averages to zero for a linear series of re-circulating loops, wherein the number of loops in the chain enhances the depolarization factor. The averaging scheme is entirely passive without the need for electronic drivers. Fiber loops in combination with a beam splitter/combiner can also be used for depolarizers as described in U.S. Patent Application No. 2003/0063833 (Gonthier, et al., Apr. 3, 2003) and in U.S. Pat. No. 6,735,350 B1 (Gauthier, May 11, 2004). A passive integrated-optics version of the loop type depolarizer is described in U.S. Pat. No. 6,891,998 (Jones, May 10, 2005) wherein a waveguide structure on a planar substrate with ancillary microresonators simulates re-circulating loops that couple evanescently light between the waveguide and the microresonators. Polarization states of the recirculated light from a microresonator are added randomly to the main beam in the waveguide; thus, with a sufficient number of resonators the output light tends to a depolarized state. Another method described in U.S. Patent Application No. 2003/0007149 (Yamamoto, Jan. 9, 2003) uses a series of birefringent plate-pairs that are bonded together such that the optical axis of each pair section is orthogonal to each other. The junction along the optical axis between the pair section is angled geometrically at 45° in order to enhance the mixing or averaging of the polarization states. The depolarization efficiency can be enhanced by inserting additional plate-pairs in the beam path; however, this occurs with a loss in beam intensity. Another application of a birefringent plate pair is described in U.S. Pat. No. 6,498,869 (Yao, Dec. 24, 2002) and U.S. Patent Application No. 2003/0112436 (Yao, Jun. 19, 2003), however, unlike Yamamoto's 45° angle junction between the plate-pairs, Yao uses a shallower angle between the plate-pair such that the phase gradient across the beam diameter is 360°. This allows the polarization states of the beam through the plate-pair to be mapped out spatially in a linear pattern symmetrically about the center-line of the beam. The beam is launched into a fiber which spatially averages the Stokes polarization parameters into a null thus resulting in a depolarized beam with only one birefringent plate-pair. Another depolarizer system has been described in U.S. Pat. No. 6,819,810 (Li, et al., Nov. 16, 2004) that utilizes 6 optical elements consisting of birefringence slabs, combiners, a Faraday rotor, and a mirror to retro-reflect a depolarized output beam from the optical ensemble for an initial irradiating polarized beam into the optical chain. It will be shown in the present invention that a far simpler method for generating depolarized light beam from a retro-reflected light beam is with a single optical element, namely, a corner cube.
A recent entry into the depolarizer category has been a corner cube as described in U.S. patent application Ser. No. 10/763,529 (Kalibjian, Jan. 23, 2004) and also in a technical journal article (R. Kalibjian, Optics Communications Vol. 240, October 2004, pp. 39-68). The corner cube depolarizer is based upon spatial averaging of the six polarization states of the retro-reflected beam from the output hexads of the corner cube. In this case a circularly polarized input light beam is normally incident to a high-refractive index corner cube (uniquely at n0=1.76748 for 633-nm wavelength). The retro-reflected light from the corner cube is launched into a fiber which spatially averages the beam into completely depolarized light at the fiber output. In the above cited patents the output beam from the various depolarizers are temporally-incoherent; however, an exception is the corner cube depolarizer which has a phase-equalized beam-director resulting in a temporally-coherent depolarized beam. The corner cube depolarizer as compared to other types has the advantage of being deployed over the entire spectral range of the glass and having negligible degree of polarization in a 100-nm bandwidth.
In summary, current depolarization methods for narrow bandwidth lasers require scrambling the polarization states either by active means in an integrated-optic waveguide or by passive means in re-circulating fiber loops or in birefringent plate pairs. Ideally, the aim in depolarization for these devices is to create a sufficient number of polarization states such that the sum of each Stokes polarization parameter cancels out to zero. On the other hand, the corner cube depolarizer is based upon an entirely different concept because of the cube's unique polarization properties. It has the advantage of having a very small degree of polarization as compared to the other types of depolarizers. The corner cube depolarizer is noteworthy in reducing the complexity in packaging by utilizing only a single small-sized corner cube as compared to the packaging of multiple loops of fiber or birefrigent plate-pairs of the previously described depolarizers. The high-refractive glass corner cube can be obtained only on special order at a high procurement cost, whereas low-refractive-index BK7 glass corner cubes are commonly available at relatively low-cost. For this reason the present depolarizer invention has been developed in order to utilize a BK7 glass corner cube which is deployed in a slightly different manner than the high-refractive-index corner cube. The present invention can utilize glass corner cubes of arbitrary refractive index irradiated by a linearly polarized light beam as contrasted to the requirement for a circularly polarized light beam for the high-refractive glass corner cube.